1. Field of the Invention
This invention relates to web shifting in a vertical direction and more particularly to shifting a print ribbon divided into horizontal colored strips, in a printer, for applying the desired color to a print medium.
2. Description of the Prior Art
A common prior art system for shifting a colored ribbon is one where the entire ribbon cartridge is shifted. A stepping motor or solenoids shift the entire ribbon cartridge to a different level to print a different color strip. This technique entails moving a tremendous amount of mass requiring a large prime mover and also is limited in accuracy due to the great distance the controlling arms are from each other because no contact or vertical guidance occurs near the printhead itself. The problem of tolerance is generally solved with a wider band or some adjustment to compensate for errors.
The entire carriage shifter may also be equipped with a steel wire that runs across the printer to keep the print ribbon from sagging in wide printers. This technique allows the user of narrower band strips but still requires an adjustment screw to compensate for molding tolerances.
Another prior art mechanism is a solenoid mounted directly on the printhead with a guide mechanism to hold the ribbon close to the solenoid to accurately control shifting. This is more accurate than the previously described prior art mechanisms because it controls the ribbon very close to the printhead. It also allows the use of strips only slightly wider than the printhead swath. However, the speed of shifting is severely limited by the mass of the solenoid. Also, there is a high power consumption in moving quickly.